This is the second revision of an application. The team has expanded slightly; the budget has remained the same; and the text has been extensively rewritten in the light of reviewers' comments. Little is known about drug use in young adulthood, although a great deal of epidemiologic and etiologic research in drug use and dependence has focused on adolescence. Young adulthood is the period in which escalation from drug use to dependence is likely to occur and when trajectories differentiating experimental from chronic drug use are defined. Since influences on transitions in young adulthood likely differ from those in adolescence, longitudinal data that extend from childhood, through adolescence and early adulthood, and into young adulthood are important in understanding natural history. Influences on transitions in stages of drug use exist at the level of the individual and the social environment, and their interactions. Important individual-level influences include genetic background and the presence of psychiatric and behavioral disorders, such as depression, alcohol disorders, antisocial behaviors and antisocial personality disorder. Important social environmental influences include engagement in adult roles, development of alternative reinforcers during young adulthood, and neighborhoods. It is likely that there are distinct pathways to drug exposure, use, dependence, and remission among those who differ on these characteristics, as well as on early history of the precursors and antecedents of these characteristics. This application is to collect and analyze interview and biological data from participants in the Prevention Research Center (PRC) study, a cohort of 2311 children in 19 Baltimore City public schools, with 10 prior waves of data since they were in first grade in 1985 and 1986 (now approaching 28-29 years of age). Prior assessments include psychiatric and behavioral problems and disorders, as well as the social environmental characteristics listed above. Proposed assessments include blood or buccal sample for DMA, and many constructs assessed in prior waves. Prior field work suggests that more than 80% of the original cohort can be located and interviewed. The population-based aspect of the cohort, and the location in Baltimore, with its high level of drug use and large African-American sample, are important advantages of this project. The timing of follow-up in young adulthood, and the richness of prior assessments, is unmatched. If interventions for prevention of drug use, and escalation to dependence and chronicity, are to be effective, the implementation must include strategies targeted for young adults where these transitions are most likely to occur. These prevention strategies can be most effective when targeting risk and protective factors for which there is credible evidence of a potential causal influence such as we propose to gather.